The prior art discloses a number of methods for removing heat from gas fluidized bed polymerization processes.
GB 1415442 relates to the gas phase polymerization of vinyl chloride in a stirred or fluidized bed reactor, the polymerization being carried out in the presence of at least one gaseous diluent having a boiling point below that of vinyl chloride. Example 1 of this reference describes the control of the temperature of polymerization by the intermittent addition of liquid vinyl chloride to fluidized polyvinyl chloride material. The liquid vinyl chloride evaporates immediately in the bed, resulting in the removal of the heat of polymerization.
U.S. Pat. No. 3,625,932 describes a process for polymerization of vinyl chloride wherein beds of polyvinyl chloride particles within a multiple stage fluidized bed reactor are kept fluidized by the introduction of gaseous vinyl chloride monomer at the bottom of the reactor. Cooling of each of the beds to remove heat of polymerization generated therein is provided by spraying liquid vinyl chloride monomer into the ascending gas stream beneath the trays on which the beds are fluidized.
GB 1398965 discloses the fluidized bed polymerization of ethylenically unsaturated monomers, especially vinyl chloride, wherein thermal control of the polymerization is effected by injecting liquid monomer into the bed using one or more spray nozzles situated at a height between 0% and 75% of that of the fluidized material in the reactor.
U.S. Pat. No. 4,390,669 relates to homo- or copolymerization of olefins by a multi-step gas phase process which can be carried out in stirred bed reactors, fluidized bed reactors, stirred fluidized bed reactors or tubular reactors. In this process polymer obtained from a first polymerization zone is suspended in an intermediate zone in an easily volatilized liquid hydrocarbon. The suspension, so obtained, is fed to a second polymerization zone where the liquid hydrocarbon evaporates. In Examples 1 to 5, gas from the second polymerization zone is conveyed through a cooler (heat exchanger) wherein some of the liquid hydrocarbon condenses (with comonomer if this is employed). The volatile liquid condensate is partly sent in the liquid state to the polymerization vessel where it is vaporized and serves to remove some the heat of polymerization. This reference is ambiguous as to how or where the liquid is introduced into the polymerization vessel.
EP 89691 relates to a process for increasing the space time yield in continuous gas fluidized bed processes for the polymerization of fluid monomers, the process comprising cooling part or all of the unreacted fluids to form a two phase mixture of gas and entrained liquid below the dew point and reintroducing said two phase mixture into the reactor. This technique is referred to as operation in the "condensing mode". EP 89691 also discloses that it is possible to form a two-phase fluid stream within the reactor at the point of injection by separately injecting gas and liquid under conditions which will produce a two phase stream.
U.S. Pat. No. 5,541,270 claims that by cooling the recycle gas stream to a temperature sufficient to form a liquid and a gas and by separating the liquid from the gas and then feeding the liquid directly to the fluidized bed, the total amount of liquid which may be reintroduced into the fluidized bed polymerization reactor for the purpose of cooling the bed by evaporation of the liquid, can be increased thereby increasing the level of cooling and thereby to achieve higher levels of productivity.